An ideal cancer therapy would be a treatment that causes all cancer cells to disappear, leaving behind only healthy, untransformed tissue. The technology of gene transfer has been proposed towards this end, whereby cancer cells themselves are harnessed to produce a fatal protein which causes their own demise. However, simply having tumor cells produce a fatal toxin is not sufficient to achieve the stated goal, however, because of lingering effects upon neighboring cells following cancer cell death. For example, the release of the toxic gene product into the extracellular space following necrotic cell death can cause a severe bystander effect resulting in harm to neighboring tissue. Thus, a more desirable process resulting in cell death is apoptosis, where cells shut down without the bystander effect.
Apoptosis, also referred to as physiological cell death or programmed cell death, is a normal physiological process of cell death that plays a critical role in the regulation of tissue homeostasis by ensuring that the rate of new cell accumulation produced by cell division is offset by a commensurate rate of cell loss due to death. Apoptosis can be characterized by morphological changes in the cell, including fragmentation of nuclear chromatin, compaction of cytoplasmic organelles, dilatation of the endoplasmic reticulum, a decrease in cell volume and alterations to the plasma membrane, resulting in the recognition and phagocytosis of apoptotic cells and prevention of an inflammatory response.
Gene delivery has been used in the past in an attempt to treat various cancers (Rubin et al. (1997) Gene Ther. 4: 419-425 and Vogelzang et al. (1994) Hum Gene Ther. 5: 1357-1370). However, independent of bystander effects, the use of many gene delivery methods adversely affects healthy, untransformed cells due to indiscriminant transfection of all cell types in a given area. A method to selectively target cancer cells for treatment is desirable, but the similarity of transformed cells to normal somatic cells makes this objective extremely difficult to achieve. Although the attachment of ligands to gene delivery complexes is a method that has yielded some progress in targeted gene delivery to normal tissues (Hood et al. (2002) Science 296: 2404-2407), this method has not produced much direct success with cancer cells because, in part, of the similarities between the receptors expressed by tumor cells and the tissues from which they originated. Commonalities between tumor cells, which are also unique to tumor cells, are continually being sought.
Previous published work includes the delivery of genes coding for caspases, which can induce apoptosis using a prostate specific composite promoter, ARR2PB2. (See Xie et al. (2001) Cancer Res. 61: 6795-6804; and Shariat et al. (2001) Cancer Res. 61: 2562-2571). This work demonstrated tissue-specific targeting and induction of apoptosis in prostate cancer cells, by placing the caspase transgene under the transcriptional control of a tissue-specific promoters, which are functional in prostate cancer cells rather than all cells.
Several pro-drug activation genes have also been studied for application in cancer gene therapy. In one example, herpes simplex virus thymidine kinase (HSV-TK) in combination with the pro-drug ganciclovir represents a prototypic pro-drug/enzyme activation system known in the art with respect to its potential applications in cancer gene therapy. HSV-TK phosphorylates the pro-drug ganciclovir and generates nucleoside analogs that induce DNA chain termination and cell death in actively dividing cells. Tumor cells transduced with HSV-TK acquire sensitivity to ganciclovir, a clinically proven agent originally designed for treatment of viral infections. (Moolten et al. (1990) Natl. Cancer Inst. 82:297-300; Ezzeddine et al., (1991) New Biol. 3:608-614). The pro-drug-TK approach was also reported by Yamamoto et al. in which the COX-2 promoter was used to reduce the level of toxicity due to thymidine kinase gene delivery in the liver ((2001) Mol Ther. 3: 385-394). However, this work relies on the complex activation of a pro-drug.
The heightened COX-2 expression in cancer cells has been used in the past as a form of cancer treatment by COX-2 inhibitors that cause inhibition of COX-2 (Elder et al. (1997) Clin Cancer Res. 3:1679-1683). COX-2 inhibitors are already on the market to combat inflammation, sold under the trade names Vioxx™ and Celebrex™. However, to date, the prior art only speaks of altering COX-2 levels, it does not address using elevated levels of COX-2 as a means of targeting a specific population of cells to induce apoptosis.
Accordingly, a need exists for specifically targeting cancer cells and inducing apoptosis in such cells. A need also exists for targeted gene therapy at the DNA level, through the use of appropriate promoters, such that entire classes of cells can be treated with one transfection.